1. Field of the Invention
The present invention relates to a production process of a liquid composition useful as an ink-jet ink, and an image forming process and an image forming apparatus using the liquid composition.
2. Description of the Related Art
In recent years, digital printing technology is making very dramatic progress. Typical examples of this digital printing technology include what is called electrophotographic technique or ink-jet technique, and its significance has been more and more increased as image forming technique in offices, homes and so forth.
In particular, the ink jet technique has such a great characteristic feature as compactness and low power consumption, as a direct recording method. Also, image quality is being rapidly improved as nozzles are made finer. An example of the ink jet technique is a method in which an ink fed from an ink tank is heated with heaters in nozzles to cause evaporation to bubbling, whereupon the ink is ejected to form images on a recording medium. Another example is a method in which the ink is ejected from nozzles by vibrating piezoelectric elements.
Water-soluble dye inks have been applied to these methods to date. However, the dye inks have involved problems of blurring or bleeding, feathering and weatherability.
In order to solve these problems, it has been investigated in recent years to use pigment inks (see U.S. Pat. No. 5,085,698). In fact, ink-jet inks containing a pigment dispersion in their ink compositions also begin to spread.
However, the pigment inks are often poor in long-term storage stability and ejection stability from an ink jet head compared with dye inks. In addition, images formed with the pigment inks generally tend to be low in color developability compared with images formed with the dye inks because they cause light scattering and/or light reflection by the influence of pigment particles.
As a method for improving the color developability of the pigment inks, it has been attempted to pulverize pigment particles. A pigment (hereinafter referred to as “fine pigment particles” pulverized to 10 nm or less suffers little influence of light scattering and increases its specific surface area and is thus expected to achieve the color developability comparable with that of a dye.
The pulverization of the pigment particles is generally mechanically conducted by means of a dispersing machine such as a sand mill, roll mill or ball mill. In these methods, the pulverization of the pigment is limited to the vicinity of primary particles (about 100 nm), and so it takes a lot of time and cost to require further pulverization. In addition, it is difficult to stably provide those having uniform quality (Japanese Patent Application Laid-Open No. H10-110111).
On the other hand, Japanese Patent Publication No. H06-096679 and Japanese Patent Application Laid-Open No. 2004-091560 have proposed a method for preparing fine pigment particles by dissolving a pigment in a solvent and then mixing the resultant solution with a poor solvent for the pigment to reprecipitate the pigment (hereinafter referred to as “reprecipitation method”).
The reprecipitation method is an effective method for pulverizing a pigment, but involves a problem that the concentration of the pigment to be prepared is extremely dilute because the solubility of the pigment in the solvent is poor.
In order to apply such fine pigment particles to, for example, a liquid composition for ink, it is essential to concentrate the fine pigment particles. However, a method making use of an evaporator or ultrafilter, which is known as a general concentrating method, takes a lot of time and cost to concentrate the dilute fine pigment particles to a high concentration.